Preparation method for positive electrode material for secondary battery

ABSTRACT

Disclosed is a method of preparing a cathode electrode material for a secondary battery, including a hydrate precursor preparation step of preparing a manganese phosphate hydrate precursor using a coprecipitation process, a synthetic powder preparation step of preparing a synthetic powder by mixing the manganese phosphate hydrate precursor in a powder form with lithium phosphate and carbon, an oxide material powder preparation step of preparing a lithium manganese phosphate oxide material powder by milling and annealing the synthetic powder, a composite powder preparation step of preparing a composite powder by mixing the lithium manganese phosphate oxide material powder with a Li 2 MnO 3 -based cathode material, and a slurry preparation step of preparing a slurry by mixing the composite powder with a conductor and a binder.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/KR2015/006172 filed on Jun. 18, 2015,which in turn claims the benefit of Korean Application No.10-2014-0115611, filed on Sep. 1, 2014, the disclosures of which areincorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to a method of preparing a cathodematerial for a secondary battery, and particularly to a method ofpreparing a cathode material for a secondary battery, which is able toprovide a cathode active material having superior reversible propertiesand high capacity.

BACKGROUND ART

With the drastic advancement of the electronic, communication andcomputer industries these days, camcorders, mobile phones, laptop PCs,etc. have made remarkable progress, and thus the demand for a lithiumsecondary battery is increasing as a power source for driving suchdevices.

The cathode active material of the lithium secondary battery mainlyincludes lithium cobalt oxide (LiCoO₂; LCO), and currently commerciallyavailable as inexpensive materials having high safety are spinel-typelithium manganese oxide (LiMn₂O₄; LMO), lithium composite metal oxide(LiMn_(1/3)Co_(1/3)Ni_(1/3)O₂; NMC) and olivine-type lithium ironphosphate oxide (LiFePO₄).

Among the above cathode active materials, lithium cobalt oxide (LCO) hashigh energy density, and is mainly used as a power source for smallappliances such as mobile phones, laptop PCs, etc. due to problems withtransition metal materials and stability problems, but is not suitablefor use in a large lithium secondary battery for electric cars requiringhigher stability.

On the other hand, spinel-type lithium manganese oxide (LMO) has a highenergy density of about 120 to 140 mAh/g, and is known to exhibitexcellent thermal stability of a cathode active material itself underovercharge and high voltage conditions due to the structural stabilityof the material, but has a structural problem in which manganese isdissolved when the battery temperature is increased to about 60° C.

Also, lithium composite metal oxide (NMC) has a high energy density ofabout 140 to 180 mAh/g, but is disadvantageous in terms of safety due toproblems with the transition metals cobalt and nickel. Thus, suchconventional materials have lower capacity than expected or are stilldangerous, and are very expensive, and hence, inexpensive materialshaving high safety and high energy density are required in order tocommercialize medium- or large-sized batteries.

Currently, a medium- or large-sized lithium secondary battery isrequired to exhibit high safety, a long lifetime, high energy densityand cost-effectiveness, and thus an olivine-type cathode active materialincluding iron is receiving attention. A typical olivine-type cathodeactive material, namely a lithium iron phosphate compound (LiFePO₄), isa cathode active material having superior electrical capacity of about150 to 160 mAh/g, but has a discharge voltage of 3.2 V to 3.4 V, whichis lower than those of other oxide-based cathode active materials and isthus unsuitable for use as a cathode active material for a medium- orlarge-sized lithium secondary battery requiring high energy density.

In contrast, the same olivine-type compound, namely a lithium manganesephosphate compound (LiMnPO₄), has a high discharge voltage of 3.8 V to4.0 V, similar to those of oxide-based materials, and thorough researchthereto is thus carried out in order to improve the characteristicsthereof.

Japanese Patent Application Publication No. 2007-119304 discloses amethod of preparing a lithium manganese phosphate compound (LiMnPO₄) ata low temperature under pressure through precipitation and reduction ofMn(OH)₂. However, the obtained lithium manganese phosphate compound(LiMnPO₄) cathode active material has a very low electrical capacity ofabout 40 mAh/g and thus poor electrochemical properties, making itimpossible to use industrially.

Also, Japanese Patent Application Publication No. 2007-48612 discloses amethod of preparing a lithium manganese phosphate compound (LiMnPO₄) byforming a material mixture and then firing the material mixturerecovered through spray drying. The obtained lithium manganese phosphatecompound (LiMnPO₄) cathode active material has an electrical capacity of92 mAh/g at a current density of 0.25 C, and the lithium manganesephosphate compound (LiMnPO₄) containing 15% carbon has 130 mAh/g at acurrent density of 0.12 C, but the amount of the cathode active materialin the cathode is excessively low, to a level of about 63%, from whichthe electrochemical properties of the cathode active material itself arenot regarded as being sufficiently improved.

DISCLOSURE Technical Problem

Accordingly, the present invention has been made keeping in mind theproblems encountered in the related art, and the present invention isintended to provide a method of preparing a cathode material for asecondary battery, wherein the cathode active material obtained bycompounding a highly crystalline nano-sized LiMnPO₄ material and anelectrochemically inactive Li₂MnO₃-based cathode material exhibitssuperior reversible properties and has high capacity.

Technical Solution

Therefore, an embodiment of the present invention provides a method ofpreparing a cathode material for a secondary battery, comprising: ahydrate precursor preparation step of preparing a manganese phosphatehydrate precursor using a coprecipitation process; a synthetic powderpreparation step of preparing a synthetic powder by mixing the manganesephosphate hydrate precursor in a powder form with lithium phosphate andcarbon; an oxide material powder preparation step of preparing a lithiummanganese phosphate oxide material powder by milling and annealing thesynthetic powder; a composite powder preparation step of preparing acomposite powder by mixing the lithium manganese phosphate oxidematerial powder with a Li₂MnO₃-based cathode material; and a slurrypreparation step of preparing a slurry by mixing the composite powderwith a conductor and a binder.

In the present invention, the hydrate precursor preparation stepcomprises: forming a 1 M metal solution by dissolving 1 mol of amanganese sulfate hydrate and 0.33 to 1 mol of phosphoric acid indistilled water; forming a 1 M aqueous solution by mixing ammonia waterand distilled water to control a pH in a reactor; performing acoprecipitation reaction by stirring the metal solution and the aqueoussolution under the condition that the pH is adjusted to 5 to 11 and astirring rate and a temperature of the reactor are maintained constant;removing impurities by repeating water washing and filtration of aprecipitate obtained through aging for 10 to 60 hr after completion ofthe coprecipitation reaction; and obtaining a manganese phosphatehydrate precursor by filtering and then drying the precipitate aftercompletion of the washing for removing the impurities.

In the present invention, the synthetic powder preparation stepcomprises: primarily heat-treating the manganese phosphate hydrateprecursor at 300° C. to 700° C. for 1 to 24 hr; mixing 1 mol of theprimarily heat-treated precursor with 0.9 to 1.3 mol of lithiumphosphate to give a precursor mixture, mixing 100 parts by weight of theprecursor mixture containing the lithium phosphate with 18 to 33 partsby weight of carbon, and performing stirring at a predetermined stirringrate for 30 min to 6 hr; making a pellet by press-molding the stirredprecursor; and secondarily heat-treating the pellet at 500° C. to 700°C. for 1 to 24 hr.

In the present invention, the oxide material powder preparation stepcomprises: milling the synthetic powder, obtained through thesecondarily heat-treating, at a predetermined stirring rate using aplanetary ball mill; and annealing the milled powder at 600° C. to 700°C. for 30 min to 2 hr in order to increase a crystallinity thereof.

In the present invention, the composite powder preparation stepcomprises mixing 100 parts by weight of the lithium manganese phosphateoxide material powder with 82 to 122 parts by weight of theLi₂MnO₃-based cathode material.

In the present invention, the slurry preparation step comprises mixing100 parts by weight of the composite powder, 5 to 22 parts by weight ofthe conductor, and 5 to 22 parts by weight of the binder.

Advantageous Effects

According to the present invention, a cathode active material, obtainedby compounding a highly crystalline nano-sized LiMnPO₄ material formedthrough coprecipitation and an electrochemically inactive Li₂MnO₃-basedcathode material, can exhibit superior reversible properties and hashigh capacity.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a process of producing a cathode material for a secondarybattery according to an embodiment of the present invention;

FIG. 2 is a graph showing the results of analysis of thermal propertiesTGA and DSC of a lithium phosphate oxide and a precursor according to anembodiment of the present invention;

FIG. 3 shows the results of TEM of a synthetic LiMnPO₄ powder accordingto an embodiment of the present invention;

FIG. 4 is a graph showing the XRD pattern of the synthetic LiMnPO₄powder depending on the heat treatment temperature in Example 1according to the present invention;

FIG. 5 is a graph showing the XRD pattern of the precursor mixture ofthe present invention, subjected to heat treatment at 700° C. in Example1, milling to a nano size in Example 2, and annealing in Example 3;

FIG. 6 is a graph showing the charge-discharge characteristics of anelectrode/coin cell using the synthetic LiMnPO₄ powder of Example 1according to the present invention;

FIG. 7 is a graph showing the charge-discharge characteristics of anelectrode/coin cell using the synthetic LiMnPO₄ powder of Example 2according to the present invention;

FIG. 8 is a graph showing the charge-discharge characteristics of anelectrode/coin cell using the synthetic LiMnPO₄ powder of Example 3according to the present invention;

FIG. 9 is a graph showing the charge-discharge cycling characteristicsof an electrode/coin cell using the synthetic LiMnPO₄ powder of Example3 according to the present invention; and

FIG. 10 is a graph showing the electrochemical properties of anelectrode/coin cell using the synthetic LiMnPO₄ powder of Example 4according to the present invention.

BEST MODE

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings so thatthe present invention is easily embodied by those skilled in the art towhich the present invention belongs. However, in the followingdescription of the principle of operation of the preferred embodiment ofthe present invention, a detailed description of known functions orconfigurations incorporated herein will be omitted when it mayunnecessarily obscure the subject matter of the present invention.

Further, the same reference numerals are used for portions havingsimilar functions and effects throughout the drawings.

It is also noted that in this specification, ‘connected/coupled’ refersto one component that may not only be directly coupled to anothercomponent but may also be indirectly coupled to another componentthrough an intermediate component. Further, when any portion ‘includes’any component, this means that the portion does not exclude othercomponents, but may further include other components unless otherwisestated.

FIG. 1 shows the process of preparing a cathode material for a secondarybattery according to an embodiment of the present invention.

According to an embodiment of the present invention, the method ofpreparing the cathode material for a secondary battery includes alithium manganese phosphate material synthesis step of producing alithium manganese phosphate (LiMnPO₄) material using a coprecipitationprocess and an electrode formation step of producing a cathode activematerial using the lithium manganese phosphate powder obtained throughthe lithium manganese phosphate material synthesis step. The lithiummanganese phosphate material synthesis step includes synthesizing amanganese phosphate hydrate (Mn₃(PO₄)₂.6H₂O) precursor through acoprecipitation process, heat-treating the manganese phosphate hydrateprecursor, subjecting the heat-treated precursor to ball milling to anano size and annealing, and forming a composite powder by mixing theannealed lithium manganese phosphate oxide material powder with aLi₂MnO₃-based cathode material (xLi₂MnO₃-(1-x)LiMO₂ (x=0.3, M=Ni, Mn,Co)).

Here, the manganese phosphate hydrate precursor synthesis step isperformed as follows. Specifically, reactants, that is, a manganesesulfate hydrate (MnSO₄.H₂O) and phosphoric acid (H₃PO₄), are provided,and 1 mol of the manganese sulfate hydrate and 0.33 to 1 mol ofphosphoric acid, and preferably, the manganese sulfate hydrate and thephosphoric acid at a molar ratio of 3:2, are dissolved in 500 ml ofdistilled water to give a 1 M metal solution, and in order to controlthe pH in the reactor, 200 ml of 5 N ammonia water, serving as acomplexing agent, is mixed with 800 ml of distilled water to afford 1000ml of a 1 mol aqueous solution.

The base material of the coprecipitation reactor has a pH of 5 to 11,and preferably a pH of 7, through mixing of ammonia water and distilledwater, and the reactor is set so as to be stirred at a stirring rate of100 rpm to 2,000 rpm, and preferably 1,000 rpm. The starting material iscontinuously titrated at a rate of 3 ml/min, and simultaneously, ammoniawater, provided as the complexing agent, is set so as to beautomatically titrated depending on changes in the pH in the reactor.Thereafter, while the stirring rate of the reactor and the reactortemperature (e.g. 20° C. to 60° C., and preferably room temperature) aremaintained uniform, the metal solution and the aqueous solution arestirred so that a coprecipitation reaction occurs.

After the completion of the coprecipitation reaction through titrationof the entire starting material, the stirrer of the reactor is operatedin a state in which a pH of 7 is maintained through automatic titrationof ammonia water, so that aging is performed for 10 to 60 hr, andpreferably 48 hr. Here, the obtained precipitate is repeatedly washedwith water and filtered several times, whereby impurities are removedfrom the precipitate. After the completion of the washing for removingimpurities, the precipitate is filtered and dried in a typical oven atabout 110° C. for 24 hr, thus obtaining a manganese phosphate (MnPO₄)hydrate precursor.

After the preparation of the manganese phosphate hydrate precursor asabove, a heat treatment process is performed as follows.

The manganese phosphate hydrate precursor is subjected to primary heattreatment (a heating rate of 1 to 10° C./min) in a furnace in an argonreducing atmosphere at 300° C. to 700° C., and preferably 500° C. for 1to 24 hr, preferably 15 hr, thus removing various impurities such asorganic materials from the manganese phosphate hydrate precursor.

After the completion of the primary heat treatment in this way, 1 mol ofthe manganese phosphate hydrate precursor and 0.9 to 1.3 mol of lithiumphosphate (Li₃PO₄), and preferably the manganese phosphate hydrateprecursor and the lithium phosphate at a molar ratio of 1:1.1, aremixed, after which the precursor mixture containing the lithiumphosphate is mixed with a carbon material. As such, 100 parts by weightof the precursor mixture containing the lithium phosphate and 18 to 33parts by weight, and preferably 25 parts by weight, of the carbonmaterial are mixed, and the carbon material may include any one selectedfrom among acetylene black, Ketjen black, conductive carbon black, andsucrose.

After the mixing of the mixture containing the lithium phosphate withthe carbon material, the precursor mixture including the carbon materialis mixed at a stirring rate of 100 rpm to 1,000 rpm (preferably 500 rpm)for 30 min to 6 hr (preferably 3 hr) using a planetary ball mill havingzirconia balls having an appropriate size received therein, and is thenpress-molded under a predetermined pressure (e.g. a pressure of 60 Mpa)at a predetermined temperature (e.g. room temperature) for apredetermined period of time (e.g. 30 min) using a uniaxial mold to makepellets, which are then subjected to secondary heat treatment (at aheating rate of 1 to 10° C./min, and preferably 5° C./min) in a furnacein an argon reducing atmosphere at 500° C. to 700° C. for 1 to 24 hr(preferably 10 hr), thereby synthesizing a lithium manganese phosphate(LiMnPO₄) material.

After the synthesis of the lithium manganese phosphate material throughthe heat treatment process as above, the heat-treated lithium manganesephosphate precursor is subjected to ball milling to a nano size and thento annealing, which are described below.

During the milling to a nano size and annealing of the synthetic powder,the lithium manganese phosphate oxide precursor, which is heat-treatedas above, is milled at a predetermined stirring rate (e.g. 400 rpm)using a planetary ball mill having zirconia balls having an appropriatesize received therein, and the milled lithium manganese phosphate powderis annealed at 600° C. to 700° C. for 30 min to 2 hr and preferably 1 hrin a furnace in an argon reducing atmosphere in order to increasecrystallinity, thereby synthesizing a lithium manganese phosphate oxidematerial powder.

After the synthesis of the lithium manganese phosphate oxide materialpowder in this way, in order to improve the electrochemical activity ofthe lithium manganese phosphate oxide material powder, 100 parts byweight of the synthesized lithium manganese phosphate oxide materialpowder and 82 to 122 parts by weight, and preferably 100 parts byweight, of a Li₂MnO₃-based cathode material (xLi₂MnO₃-(1-x)LiMO₂ (x=0.3,M=Ni, Mn, Co)) are uniformly mixed, thus obtaining a composite powder.

Meanwhile, after the preparation of the lithium manganese phosphatecomposite powder as above, the lithium manganese phosphate compositepowder, serving as a cathode active material, is mixed with a conductorat a predetermined weight ratio and transferred into a vessel of aslurry preparation mixer, after which a binder is titrated in anappropriate amount to the mixture in the vessel. Here, the synthesizedcathode active material, the conductor and the binder are used in amanner in which 100 parts by weight of the synthesized cathode activematerial, 5 to 22 parts by weight, and preferably 12.5 parts by weight,of the conductor, and 5 to 22 parts by weight, and preferably 12.5 partsby weight, of the binder are mixed. The binder is polyvinylidenefluoride (PVDF 8 wt %).

The slurry preparation mixer is used so that the mixture is stirred at arate of 2,000 rpm for 30 min to give a slurry, and the slurrypreparation mixer is operated for 5 min, viscosity measurement isrepeated five to six times, and stirring for a total of about 30 min isperformed, thereby adjusting the viscosity of the slurry.

During the preparation of the slurry by stirring the mixture asmentioned above, it is important to maintain the optimal conditions soas not to change the viscosity of the mixture in the stirrer or theproperties thereof due to the generation of heat upon the operation ofthe stirrer. In order to realize the optimal conditions, the stirringtime of the slurry mixture and the kind and size of balls in the stirrerhave to be optimized. In the present invention, the usage time ofzirconia balls is limited to a minimum of 5 min in order to suppresschanges in the properties (viscosity) of the slurry mixture in thestirrer.

The slurry thus prepared is subjected to casting on a piece of aluminum(Al) foil having a thickness of 20 μm to form a film. Here, when theslurry is manufactured in the form of a film, it is uniformly appliedwith a predetermined force in a predetermined direction. The electrodecoated with the slurry is sufficiently dried in a typical oven at 110°C., and the thickness of the electrode is adjusted to fall within therange of 20 μm to 40 μm. The electrode is then pressed using a pressingmachine (a roll press) to a final thickness of 15 μm to 30 μm, andpreferably 15 μm to 20 μm.

Meanwhile, the electrode having the thickness adjusted through pressingis punched so as to be suitable for a coin cell size in a dry room, andis then sufficiently dried in a vacuum oven at 80° C. for 4 hr.

MODE FOR INVENTION EXAMPLE 1

As reactants, 0.6 mol of MnSO₄.H₂O and 0.4 mol of H₃PO₄ were dissolvedin 500 ml of distilled water, thus preparing a 1 M metal solution, and 1M ammonia water was provided as a complexing agent. 500 ml of distilledwater was placed in a reactor, and room temperature, pH 7, and astirring rate of 1000 rpm were set. The metal mixture was titrated at arate of 3 ml/min, and simultaneously, ammonia water was set so as to beautomatically titrated to maintain the pH in the reactor at 7. After thetermination of titration, aging was performed for 48 hr, and theobtained precipitate was washed with water and filtered to removeimpurities, followed by overnight drying in a typical oven at 110° C.,thus preparing a manganese phosphate hydrate precursor.

The prepared precursor was subjected to primary heat treatment in anargon gas atmosphere at a heating rate of 5° C./min to 500° C. for 15hr. The heat-treated precursor was mixed with Li₃PO₄ at a molar ratio of1:1.1, and acetylene black was added in an amount of 20 wt % based onthe weight of the heat-treated precursor, after which the resultingmixture was mixed at a stirring rate of 500 rpm for 3 hr using aplanetary ball mill having zirconia balls having diameters of 10 mm and5 mm mixed at a number ratio of 1:1, and then subjected to secondaryheat treatment at each of 500° C., 600° C. and 700° C. for 10 hr at aheating rate of 5° C./min in an argon gas atmosphere, thus obtaining aLiMnPO₄/C powder.

The synthesized cathode material, namely the powder having a secondaryparticle size of 50 μm or less, was used to prepare the slurry, and wasalso applied to manufacture an electrode and a cell. The cathode activematerial LiMnPO₄/C, a conductor, and a binder were mixed at a ratio of80:10:10 wt % at a rate of 2,000 rpm for 30 min using a mixer, thuspreparing a slurry.

The slurry thus prepared was cast in the form of a film on a piece ofaluminum foil having a thickness of 20 μm, immediately after which theelectrode coated with the slurry was sufficiently dried in a typicaloven at 110° C. The thickness of the sufficiently dried electrode wasadjusted to about 30 μm. The electrode was then pressed using a pressingmachine (a roll press) to a final thickness of about 15 μm. Theelectrode thus pressed was punched so as to be suitable for a cell sizein a dry-room atmosphere, and was then sufficiently dried in a vacuumoven at 80° C. for 4 hr.

EXAMPLE 2

An electrode was manufactured in the same manner as in Example 1, withthe exception that, in order to efficiently mill the powder finallyheat-treated at 600° C. and 700° C. in Example 1, a planetary ball millhaving zirconia balls having appropriate sizes and conditions receivedtherein, that is, the balls having diameters of 10 mm, 5 mm, 2 mm and 1mm at a weight ratio of 2:1:3.3:3.3, was used, and the synthesizedcathode material and the zirconia balls were provided at a weight ratioof 1:20, and thus ball milling was performed six times for 30 min eachat a predetermined stirring rate (e.g. 400 rpm).

EXAMPLE 3

An electrode was manufactured in the same manner as in Example 2, withthe exception that the synthesized cathode material obtained through themilling of Example 2 was additionally annealed at each of 600° C. and700° C. for 1 hr in an argon atmosphere. As such, the annealing wasperformed at the same temperature as the secondary heat treatmenttemperature, as shown in Table 1 below.

In order to evaluate the electrochemical properties of the electrodes ofExamples 1 to 3, a coin cell and a 3-electrode cell were fabricated. Thecoin cell was a 2032-standard coin cell, configured such that a lithiummetal serving as an anode, a PE separator, and an electrolyte solutioncomprising 1 mol of LiPF₆ dissolved in a mixed solvent of ethylenecarbonate (EC) and diethyl carbonate (DMC) (volume ratio of 1:1) wereassembled, and the 3-electrode cell was configured such that thesynthesized cathode material acted as a working electrode (WE), Li metalwas used as a counter electrode (CE) and a reference electrode (RE), andthe same electrolyte solution and separator as in the coin cell wereused.

TABLE 1 Precursor Secondary Tertiary and primary heat Ball heat heattreatment treatment milling treatment Example 1 Coprecipitation 500° C.No No (pH 7), 500° C. 600° C. No No 700° C. No No Example 2 The same600° C. Yes No 700° C. Yes No Example 3 The same 600° C. Yes Yes (600°C.) 700° C. Yes Yes (700° C.)

The thermal properties of the mixture of the MnPO₄ precursor, obtainedthrough coprecipitation of the present invention, and the lithium source(lithium phosphate) in the Examples were measured. As shown in FIG. 2,an exothermic peak was observed at about 480° C., and further weightreduction did not occur at 500° C. or more but crystallizationprogressed. That is, taking into consideration the crystallization ofthe synthetic powder of the present invention and the properties of thenanoparticles, heat treatment is preferably carried out in thetemperature range of 500° C. to 700° C.

When the heat treatment was performed at 600° C., as in Example 1, thesynthetic LiMnPO₄ powder was configured such that the primary particleswere aggregated as shown in the left of FIG. 3, the particle size beingabout 60 nm to 100 nm, and the bulk portion of the particles and theportion coated with carbon to a thickness of about 2 nm to 5 nm wereobserved, as shown in the right of FIG. 3.

In Example 1, the mixture of the MnPO₄ precursor obtained throughcoprecipitation and the lithium source (lithium phosphate) washeat-treated at each of 500° C., 600° C. and 700° C., as shown in FIG.4, and the XRD pattern of each sample was analyzed. The mixture wasdetermined to be an orthorhombic crystal structure of a Pnma spacegroup, and there were no great changes in lattice constant depending onthe heat treatment temperature, but the crystallite size andcrystallinity varied, as seen in Table 2 below. As the heat treatmenttemperature was increased from 500° C. to 700° C., the crystallite sizeand crystallinity were increased, but became similar at heat treatmenttemperatures of 600° C. and 700° C.

TABLE 2 Crystallite size Crystallinity Example 1 (nm) (%) 500° C. 30.4489.33 600° C. 64.14 94.30 700° C. 62.63 94.34

Based on the results of analysis of XRD pattern of the powderheat-treated at 700° C. (Example 1), the powder (Example 2) obtained bysubjecting the powder under the same conditions as Example 1 toplanetary ball milling, and the powder (Example 3) obtained by annealingthe powder of Example 2 at the same temperature, as shown in FIG. 5, noexternal changes in peak appear, but in Example 2, the main peaks wererelatively wide. As for the crystallite size and crystallinity based onRietveld analysis, the powder particles were milled to a nano size usinga planetary ball mill and became non-crystalline (i.e. amorphous), andthus, as shown in Table 3 below, the crystallite size and crystallinitywere reduced in the powder (Example 2) obtained by subjecting the powderunder the same conditions as Example 1 to planetary ball milling,compared to the powder heat-treated at 700° C. (Example 1). As isapparent from Table 3, the powder heat-treated at 700° C. had acrystallite size of 62.63 and a crystallinity of 94.34%, but thecrystallite size and crystallinity of the powder subjected to planetaryball milling (Example 2) were decreased to 29.93 and 83.39%,respectively. Also, when the powder (Example 2) obtained by subjectingthe powder under the same conditions as Example 1 to planetary ballmilling was annealed at 700° C., as shown in Table 3, the crystallitesize and crystallinity of Example 3 were 47.33 and 93.16%, respectively,which are higher than Example 2.

TABLE 3 Crystallite size Crystallinity Sample (nm) (%) Example 1 62.6394.34 Example 2 29.93 83.39 Example 3 47.33 93.16

Meanwhile, the powder of Example 1, heat-treated at each of differentheat treatment temperatures, was fabricated into an electrode/coin cell,and the charge-discharge performance thereof was measured underconditions of CV/CC=4.5V, 0.05 C/0.02 C charge, and 0.05 C discharge. Asshown in FIG. 6, the capacity of each sample was 58 mAh/g at a heattreatment temperature of 500° C., 69 mAh/g at 600° C., and 100 mAh/g at700° C., and the voltage was maintained at about 3.9 V. Accordingly, thecathode material powder manufactured according to the present inventionexhibited the greatest capacity at 700° C., at which crystallinity,rather than the crystallite size, was stably maintained at a high level.

The powder of Example 2 was fabricated into an electrode/coin cell, andthe charge-discharge performance thereof was measured under conditionsof CV/CC=4.5V, 0.05 C/0.02 C charge, and 0.05 C discharge. As shown inFIG. 7, both the powder at 600° C. and the powder at 700° C. wereincreased in capacity compared to Example 1. That is, the capacity ofthe powder at 600° C. of Example 1 was 69 mAh/g, but the capacity of thepowder at 600° C. of Example 2 was 81 mAh/g, and the capacity of thepowder at 700° C. of Example 1 was 100 mAh/g, but the capacity of thepowder at 700° C. of Example 2 was 120 mAh/g. The reason why thecapacity was increased as above is that, as illustrated in FIG. 5, thecrystallite size is decreased through ball milling, thus improvingperformance but decreasing crystallinity, whereby limitations areimposed on increasing the capacity.

The powder of Example 3, obtained by annealing each of the powder at600° C. and the powder at 700° C. of Example 2, subjected to planetaryball milling, at 600° C. and 700° C., was fabricated into anelectrode/coin cell, and the charge-discharge performance thereof wasmeasured under conditions of CV/CC=4.5V, 0.05 C/0.02 C charge, and 0.05C discharge. As shown in FIG. 8, the powder at 600° C. and the powder at700° C. had capacities of 130 mAh/g and 145 mAh/g, respectively, and thedischarge potential was 3.9 V at 600° C., and a flat potential of 3.8 Vwas obtained at 700° C. In particular, the powder at 700° C. exhibitednot only superior discharge capacity but also a flat discharge voltagerange, which was very long and stable.

On the other hand, the powder of Example 3, annealed at 700° C., wasfabricated into an electrode and a coin cell, and the cyclingcharacteristics thereof were measured under conditions of CV/CC=4.5V,0.05 C/0.02 C charge, and 0.1 C discharge. As shown in FIG. 9, acapacity of about 127 mAh/g was obtained at a discharge capacity of 0.1C. When the discharge current density was increased from 0.05 C to 0.1C, about 90% of the capacity at the discharge current density of 0.05 Cwas obtained. In the case where 25 cycles were maintained under theabove conditions, the charge voltage was decreased and the dischargevoltage was increased depending on the number of cycles. When 25 cycleswere maintained, the discharge voltage was set to about 4.0V, and thedischarge capacity was not decreased, but was maintained stable.

EXAMPLE 4

An electrode was manufactured in the same manner as in Example 3, withthe exception that the composite powder, comprising the synthesizedcathode active material annealed at each of 600° C. and 700° C. inExample 3 and the Li₂MnO₃-based oxide at a weight ratio of 50:50 (wt %),was used, and the thickness of the electrode was adjusted to about 20μm.

In order to evaluate the electrochemical properties of the electrode ofExample 4, a coin cell and a 3-electrode cell were fabricated. As such,the coin cell was a 2032-standard coin cell configured such that alithium metal serving as an anode, a PE separator, and an electrolytesolution comprising 1 mol of LiPF₆ dissolved in a mixed solvent ofethylene carbonate (EC) and diethyl carbonate (DMC) (volume ratio 1:1)were assembled, and the 3-electrode cell was manufactured in a manner inwhich the synthesized cathode material was used as a working electrode(WE), Li metal acted as a counter electrode (CE) and a referenceelectrode (RE), and the same electrolyte solution and separator as inthe coin cell were used.

The composite cathode material comprising the synthetic powder annealedat each of 600° C. and 700° C. and the Li₂MnO₃-based cathode material ata weight ratio of 1:1 was fabricated into an electrode and a coin cell,and the charge-discharge performance thereof was measured underconditions of CV/CC=4.6V, 0.05 C/0.02 C charge, and 0.05 C discharge. Asshown in FIG. 10, the powder at 600° C. and the powder at 700° C. hadhigh capacities of 175 mAh/g and 205 mAh/g, respectively. However, asshown in FIG. 10, the discharge voltage (i.e. average about 3.6 V) wasvery different from that of the pure phosphate-based LiMnPO₄, and thetotal energy density was increased by about 30% compared to the powderat 700° C. of Example 2. That is, the energy density of the powder at700° C. of Example 2 was 551 Wh/g (145 mAh/g×3.8 V), but the energydensity of the powder at 700° C. of Example 4 was 718 Wh/g (205mAh/g×3.5 V), representing an increase of about 30%.

The method of manufacturing the cathode material for a secondary batteryaccording to the embodiment of the present invention includes forming ahighly crystalline nano-sized LiMnPO₄ material through coprecipitationand then compounding the highly crystalline nano-sized LiMnPO₄ materialwith an electrochemically inactive Li₂MnO₃-based cathode material,thereby providing an excellent cathode active material having an energydensity ranging from a minimum of 551 Wh/g to a maximum of 718 Wh/g.

While the present invention has been described in connection with whatis presently considered to be the most practical and preferredembodiment, it is to be understood that the invention is not limited tothe disclosed embodiments. It will be apparent to those skilled in theart that various modifications and variations can be made in the presentinvention without departing from the spirit of the invention. Therefore,the scope of the present invention should not be limited to theembodiments described, but should be defined by the following claims aswell as equivalents thereof.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, a highlycrystalline nano-sized LiMnPO₄ material is formed throughcoprecipitation and is then compounded with an electrochemicallyinactive Li₂MnO₃-based cathode material, whereby the cathode activematerial can exhibit superior reversible properties and high capacity.

The invention claimed is:
 1. A method of preparing a cathode materialfor a secondary battery, comprising: a hydrate precursor preparationstep of preparing a manganese phosphate hydrate precursor using acoprecipitation process; a synthetic powder preparation step ofpreparing a synthetic powder by mixing the manganese phosphate hydrateprecursor in a powder form with lithium phosphate and carbon; an oxidematerial powder preparation step of preparing a lithium manganesephosphate oxide material powder by milling and annealing the syntheticpowder; a composite powder preparation step of preparing a compositepowder by mixing the lithium manganese phosphate oxide material powderwith a Li₂MnO₃-based cathode material; and a slurry preparation step ofpreparing a slurry by mixing the composite powder with a conductor and abinder.
 2. The method of claim 1, wherein the hydrate precursorpreparation step comprises: forming a 1 M metal solution by dissolving 1mol of a manganese sulfate hydrate and 0.33 to 1 mol of phosphoric acidin distilled water; forming a 1 M aqueous solution by mixing ammoniawater and distilled water to control a pH in a reactor; performing acoprecipitation reaction by stirring the metal solution and the aqueoussolution under a condition that the pH is adjusted to 5 to 11 and astirring rate and a temperature of the reactor are maintained constant;removing impurities by repeating water washing and filtration of aprecipitate obtained through aging for 10 to 60 hr after completion ofthe coprecipitation reaction; and obtaining a manganese phosphatehydrate precursor by filtering and then drying the precipitate aftercompletion of the water washing for removing the impurities.
 3. Themethod of claim 1, wherein the synthetic powder preparation stepcomprises: primarily heat-treating the manganese phosphate hydrateprecursor at 300° C. to 700° C. for 1 to 24 hr; mixing 1 mol of theprimarily heat-treated precursor with 0.9 to 1.3 mol of lithiumphosphate to give a precursor mixture, mixing 100 parts by weight of theprecursor mixture containing the lithium phosphate with 18 to 33 partsby weight of carbon, and performing stirring at a predetermined stirringrate for 30 min to 6 hr; making a pellet by press-molding the stirredprecursor; and secondarily heat-treating the pellet at 500° C. to 700°C. for 1 to 24 hr.
 4. The method of claim 3, wherein the oxide materialpowder preparation step comprises: milling the synthetic powder,obtained through the secondarily heat-treating, at a predeterminedstirring rate using a planetary ball mill; and annealing the milledsynthetic powder at 600° C. to 700° C. for 30 min to 2 hr in order toincrease a crystallinity thereof.
 5. The method of claim 1, wherein thecomposite powder preparation step comprises mixing 100 parts by weightof the lithium manganese phosphate oxide material powder with 82 to 122parts by weight of the Li₂MnO₃-based cathode material.
 6. The method ofclaim 1, wherein the slurry preparation step comprises mixing 100 partsby weight of the composite powder, 5 to 22 parts by weight of theconductor, and 5 to 22 parts by weight of the binder.